As a device that detects a substance (for example, a gas) which is difficult to be visible by performing imaging with visible light in a detection region in the prior art, a gas concentration measuring device that emits measurement light from a semiconductor laser toward a measurement environment has been known (for example, see PTL 1).
The gas concentration measuring device in PTL 1 emits, from a semiconductor laser unit, the measurement light subjected to frequency stabilization with a gas absorption line and causes a light receiver to receive reflected measurement light reflected from the measurement environment with the emission of the measurement light. The gas concentration measuring device detects a fundamental wave phase-sensitive detection signal and a second harmonic phase-sensitive detection signal having a modulation frequency of a semiconductor laser from an output signal of the light receiver and measures a gas concentration of the measurement environment based on a ratio of both signals. In addition, in the semiconductor laser unit of the gas concentration measuring device, the light receiver is disposed deep inside on a center axis line in a bottomed main body having a cylindrical shape. A semiconductor laser module including a semiconductor laser, a laser pointer that emits visible light as guide light, and combination wave means, which is disposed on an optical axis of a light receiving device and substantially coaxially combines the measurement light that is emitted from the semiconductor laser and guide light that is emitted from the laser pointer, are installed in the main body. In this manner, the gas concentration measuring device is capable of checking an emission position of the measurement light and emitting the measurement light toward a measurement position with accuracy.
In addition, a gas detecting device that performs gas detection on a predetermined point has been known (see PTL 2). The gas detecting device causes laser light emitted from a laser diode (LD) module to diverge into measurement light and reference light with a half mirror. The measurement light is used for the gas detection. The reference light is received by a wavelength processing light receiver after passing through a gas cell. A wavelength processing controller performs a wavelength checking process or a wavelength correcting process of the LD module based on a detection signal obtained by the received reference light.
In this disclosure, it is possible to easily detect a substance in a detection region. Hence, in this disclosure, it is possible to easily detect, with fine resolution, a substance that is difficult to be visible through imaging with visible light in the detection region, without significant effort of a user.